For a product of multiple-layer structure, it is advantageous to coat multiple layers simultaneously to improve manufacturing productivity. For imaging elements, particularly photographic silver halide imaging elements, the layers are typically coated employing multilayer slide bead coating processes such as described in U.S. Pat. No. 2,716,419 and multilayer slide curtain coating processes such as described in U.S. Pat. No. 3,508,947. The key factors allowing the coating of multiple layers without intermixing are twofold, high viscosity of the coating solution and the ability to immobilize solutions quickly. An aqueous gelatin solution has a unique thermally reversible gelation property, it is liquid above its gelation temperature, and it gels below its gelation temperature (ref. Chapter 2, The Theory of the Photographic Process, by T. H. James). This unique property is often referred to as “chill-setting”, and is important and useful to provide the crucial first stage of the post-coating process, i.e. locking the layer-structure in place and giving it mechanical stability to perturbation during drying. Gelatin is generally used in every coating layer as the vehicle to allow chill-setting to occur quickly in photographic products. The temperature for chill-setting to occur depends on the type of gelatin, gelatin concentration, pH, ionic strength of coating solution, other components in the solution, time, and various other factors, generally ranging from 30° C. to 0° C.
In recent years various dry-type image-imaging processes which utilize a color-forming component capable of generating visible images by coloration or discoloration reaction have been disclosed in the patent literature. These imaging processes do not use a liquid developing solution or the like and, therefore, do not generate wastes. Both light sensitive and heat developable and light sensitive and pressure developable processes have been discussed in great detail. Both processes utilize a photopolymerization composition to create a latent image by irradiating the imaging element with light through an image original or using a digital image file. The latent image is composed of domains exposed to light at different degrees (from unexposed to fully exposed areas). The fully exposed domains have the highest degree of hardening and the unexposed domains have lowest degree of hardening. Under heat or pressure or both, a visible image is formed due to the difference in the mobility of the color-forming component, said mobility being controlled by the degree of hardening. For example, in the unexposed area the color-forming component can move freely to allow a color formation reaction and in the fully exposed area the color-forming component cannot move, thereby inhibiting a color formation reaction.
Imaging systems employing microencapsulated radiation sensitive compositions have been disclosed in U.S. Pat. Nos. 4,399,209; 4,416,966; 4,440,846; 4,766,050; and 5,783,353. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable composition in the internal phase is image-wise exposed to light. In the most typical embodiments, the photohardenable composition is a photopolymerization composition including a polyethylenically unsaturated compound and a photoinitiator. A color former is encapsulated with the photopolymerization composition. Exposure to light hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is developed by subjecting it to a uniform rupturing force in the presence of a developer.
An image transfer system in which the developer material is coated on a separate substrate as a separate developer or copy sheet is disclosed in U.S. Pat. No. 4,399,209. A self-contained imaging system in which the encapsulated color former and the developer material are present in one layer or in two interactive layers is disclosed in U.S. Pat. No. 4,440,846. Self-contained imaging systems having an opaque support are disclosed in commonly assigned U.S. Pat. No. 6,080,520. A two-sided imaging material is disclosed in commonly assigned U.S. Pat. No. 6,030,740.
The imaging system is capable of providing a full color imaging material in which the microcapsules are in three sets containing cyan, magenta, and yellow color formers respectively sensitive to red, green, and blue light. For good color balance, the light sensitive microcapsules are sensitive (X max) at about 450 nm, 540 nm, and 650 nm, respectively. Such a system is useful with visible light sources in direct transmission of reflection imaging. It is further useful in making contact prints, projected prints of color photographic slides, or in digital printing. It is also useful in electronic imaging using lasers or pencil light sources of appropriate wavelengths. Because digital imaging systems do not require the use of visible light, sensitivity can be extended into the UV and IR to spread the absorption spectra of the photoinitiators and avoid cross talk.
U.S. Pat. No. 5,783,353 discloses a self contained imaging system wherein the imaging layer is sealed between two supports to form an integral unit (laminated structure). The sealed format is advantageous in that it can reduce oxygen permeation and improve stability of the media. U.S. Pat. No. 6,365,319 discloses a self-contained imaging assembly which has an imaging layer containing developer and photohardenable microcapsules placed between two support members, wherein one support is transparent and one support is opaque, and comprises a metallic barrier layer, and exhibits a water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in2/day). U.S. Pat. No. 6,544,711 B1 discloses a self-contained imaging system which has an imaging layer containing developer and photohardenable microcapsules placed between two support members, wherein at least one support is transparent and at least one support comprises a ceramic barrier layer and exhibits a water vapor transmission rate not more than about 0.47 g/m2/day (0.03 g/100 in2/day). While the laminated structure has improved media stability and protection against damage, the clear over-laminate through which one views the image degrades image sharpness and resolution. In addition, the laminated structure adds complexity and cost to manufacture.
U.S. Application 2002/0045121 A1 discloses a self-contained photosensitive material which includes an imaging layer of photosensitive microcapsules and a developer on a support and a protective coating oil the imaging layer. The protective coating comprises a water-soluble or water-dispersible resin and provides scratch resistance and water resistance to the imaging media. The protective coating may also include a cross-linking agent, UV absorbing compounds, and pigments. Such elements have a number of disadvantages: The element has inherently low surface gloss. It requires careful handling to avoid accidentally rupturing the photosensitive microcapsules prior to exposure. In addition, such an element is prone to scratches, pressure marks, and cinch marks during manufacturing winding, rewinding, and finishing operations.
It is difficult to coat multiple layers which do not contain gelatin or other chill-settable materials because coating solutions mix and the intended layered structure is not obtained. It is easy, but not desirable, to coat one layer at a time, due to the excessive time and energy required on the coating machine. As new imaging elements which utilize non-gelation layers are developed, it is becoming crucial to develop efficient and effective methods for coating such materials productively in a manufacturing facility.